Combination of 2, 4, 6-trialkylphenols and mono-ethers of dihydric phenols as rubber antioxidants



ilnieed States Patent (3 COMBINATION OF 2,4,6-TRIALKYLPHENOLS AND MGNQ-ETHERS OF DEYDRIC PHENQLS AS RUBBER ANTIOXIDANTS Gordon Stansfieid Mills, Rnabon, and Henry W. H. Robinson, Deeside, Llangollen, Wales, assignors to Monsanto Chemicals Limited, London, England, a British company No Drawing. Application February 24, 1953, Serial No. 338,604

Claims priority, application Great Britain February 27, 1952 11 Claims. (Cl. 260-4535) This invention relates to antioxidants for rubber, and especially to antioxidants which do not seriously discolour rubber stocks in which they are used and which are therefore suitable for use in White and other lightcolc-ured compositions.

it has long been known that vulcanized rubber is liable to deteriorate on exposure to the action of heat, light and oxygen-containing gases, and that deterioration occurs in the atmosphere under normal conditions of use. To resist this effect various substances have been proposed for use in rubber as antioxidants, but many of these suffer from the disadvantage that they cause darkening with time and badly discolour the rubber stocks in which they are incorporated if these should be of a light colour. This problem does not arise with dark-coloured stocks obtained using carbon blacks, for instance, but it is a serious difficulty in such compositions as the White rubber stocks obtained using zinc oxide, titanium dioxide and analogous substances.

in determining the effect of rubber antioxidants there are two standard tests, the first being the action of oxygen under pressure on a vulcanised rubber prepared using the antioxidant (the so-called oxygen bomb test) and the second the behaviour of the sample of vulcani-sed rubber on heating in a hot air oven (the hot air oven test). For an antioxidant to be really satisfactory it is necessary for it to give good results in both of these tests, and even among the limited number of antioxidants which do not badly discolour light-coloured rubber stocks only a few are capable of giving adequately good results in both of them.

it has now been found that an efiective antioxidant action can be obtained by using as an antioxidant in vulcanised rubber a mixture of a 2z4c6-trialkylphenol and a mono-ether of a dihydric phenol, and moreover that mixtures of this kind are suitable for use in light-coloured stocks. Neither of these two types of compound-s alone will give satisfactory results in both the oxygen bomb test and the hot air oven test, but by using the two in conjunction eminently satisfactory results can be obtained in both tests, as will be seen from the figures given later in this specification.

The 2:4:6-trialkyl-phenol used is preferably a 2:4- dimethyl-6-actyl-phenol (especially one obtained bythe reaction of 2:4-dimethyl-phenol with diisobutylene), and other trialkyl-phenols which can be used are 2:6-dimethyl-4-octyl phenol (again one obtained from the dimethyl-phenol and diisobutylene is very suitable) and 2:6-dibutyll-methyl-phenol, in particular Where the butyl group is tertiary butyl. It is not necessary to use a pure 2z4r6-trialkyl phenoi in preparing mixtures of the 2,789,108 Patented Apr. 16, 1957 above kind, and for instance an alkylated commercial cresylic acid can be used in admixture with the monoether. Thus an octylated cresylicacid obtained by octylating the acid with diisobutylene can very suitably be used in conjunction with the mono-ether; such an octylated product might contain several isomeric 2:4:6-trialkyl-phenols along with other substances.

The mono-ether of a dihydric phenol used according to the invention .is preferably the mono-benzyl ether, though other others, for instance the mono-ethyl ether, are also suitable. In practice a mono-ether of a commercial mixture of dihydric phenols is best used, especially one containing a preponderance of catechol, and very good results have been obtained using the monoethers, particularly the mono-benzyl ether, of a commercial mixture of dihydric phenols containing about 90% by weight of catechol and methyl-substituted catechols. equal proportions of isomeric dihydric phenols can also be used in forming the ether.

The proportions. of the two constituents of the antioxidant mixture, the 2:4:6-trialkyl-phenol on the one hand and the mono-ether on the other, can lie between Wide limits. Good results can be obtained using mixtures containing these in proportions from 25:75. to. :25 by Weight, and also mixtures outside these limits. Very satisfactory results can be obtained using approxi mately equal proportions by Weight of the trialkyl-phenol and the mono-ether, but on the Whole, taking all the fac tors into account (including the inevitable discolouration of the rubber on ageing) it is preferable to have rather more of the 2:4:6-trialkyl-phenol present than the monoether. Mixtures containing these approximately in the proportions 75:25 by weight are probably to be preferred. Thus in the particular instance of a mixture of 2:4- dimethyl-6-octyl phenol and the mono-benzyl ether of the mixture of dihydric phenols containing about by Weight of catechol and methyl-substituted catechols referred to above the 75:25 mixture has been found generally preferable to either a 50:50 or a 25:75 mixture.

It will be appreciated that the antioxidantsof the invention can be used with dark-colouregl rhbber stocks also, but they are of particular value with light-coloured stocks owing to the limited number of satisfactory antioxidants available which do not discolour such stocks unduly. They are of value both with natural and synthetic rubber.

It will of course be understood that while the invention has been described with reference to the use of a mixture of antioxidants, the constituents of the mixture may be incorporated in the rubber separately. This is indicated in the claims. In general, however, it will be found more convenient to mix the constituents together before use.

The invention is illustrated by the following examples:

EXAMPLE 1 A rubber stock of the following composition was compounded on a laboratory mill:

Parts by weight Accelerator (tetramethyl thiuram disulphide) 0375 Other commercial mixtures containing more I The stock was first made up without any antioxidant, v and then with 1 part by weight of a mixture of equal quantities by weight of 2:4-dimethyl--octyl-phenol (obtained by the reaction of 2:4-dirnethyl-phenol with diisobutylene) and of the mono-benzyl ether of a commercial mixture of dihydric Iphenols containing about 9% by weight of catechol and methyl-substituted catechols. Eachof the two stocks was vulcanised at 126 C. for

'20 minutes, and the physical properties of the vulcanised Oxygen bomb test (1) TENSION STRENGTH (KGMSJSQ. GM.)

After 4 Percentage Original days of original figure Stock without antioxidantnh 246 149 61 Stock with antioxidant 228 194 (ii) PERCENTAGE ELONGATION AT BREAK Stock without antioxidant 720 623 87 Stock with antioxidant 720 645 89 (iii) MODULUS AT 500% ELONGATION (KGMSJSQ. CM.)

Stock without antioxidant 91 84 92 Stock with antioxidant 82 98 119 Hot air oven test (1) TENSION STRENGTH (KGMSJSQ. OM.)

After 7 Percentage Original days of original figure Stock without antioxidant 246 150 Stock with antioxidant 228 184 81 (ii) PERCENTAGE ELONGATION AT BREAK Stock without antioxidant 720 578 80 Stock with antioxidant 720 598 83 7 (iii) MODULUS AT 500% ELONGATION- (KGMSJSQ. CM.)

Stock without antioxidant 91 97 106 Stock with antioxidant 82 107 130 In order to test any discolouring effect of the antioxidant the two vulcanised stocks were subjected to the action of ultraviolet light for a period of five hours, and the discolouration at the end of the period was noted, as follows:

Stock without antioxidant Exceedingly pale yellow. Stool: with antioxidant Very pale yellow.

EXAMPLE 2 The tests carried out in Example 1 were repeated using Mixtures containing these respectively in the proportions 25 :75, 50:50 and 75:25 by weight were used, and in this instance the physical properties of the vulcanised rubbers obtained were determined before ageing and (a) after ageing for 4 days at C. in an oxygen bomb at a pressure of 300 lbs/sq. inch; (b) after ageing for 8 days at 70 C. in a hot air oven.

The following results were obtained. It is to be observed that the results using the 50:50 mixture are slightly diiferent from those of Example 1, as a diiierent series of tests was involved and there was of course the usual variation of results from batch to batch as was to be expected.

Oxygen bomb test (i) TENSION STRENGTH (KG-MSJSQ. CM.)

After 4 Percentage Original days of original figure Stock with 25:75 mixture as antioxian 235 198 84 Stock with 50:50 mixtureas antioxidant 221 200 90 Stock with :25 mixture as antioxidaut 232 205 88 (ii) PERCENTAGE ELONGATION AT BREAK Stock with 25:75 mixture as antioxidant 715 655 91 Stock with 50-50 mixture as anti 1- dent- 700 640 91 Stock wit 75:25 mixture as antioxidant 705 640 (iii) MODULUS AT 500% ELONGATION (KGMSJSQ. CM.)

Stock with 25:75 mixture as antioxi dant 89 98 Stock with 50:50 mixture as antioxidant 86 103 119 Stock with 75:25 mixture as antioxidant 92 109 113 Hot air oven test (i) TENSION STRENGTH (KGMSJSQ. OM.)

After 8 Percentage Original days of original figure Stock with 25=75 mixture as antioxidant 235 186 79 Stock with 50:50 mixture as antioxidant 221 204 92 Stock with 75:25 mixture as antioxidant 232 187 80 (ii) PERCENTAGE ELONGATION AT BREAK Stock with 25:75 mixture as antioxidant 715 580 81 7 (iii) MODULUB AT 500% ELONGATION (KGMSJSQ. OM.)

Stock with 25:75 mixture as antioxidant 89 119 134 Stock with 50:50 mixture as antioxidant 86 117 136 Stock with 75:25 mixture as autioxidant 92 118 128 EXAMPLE 3 The tests carried out in Example 1 were repeated using the same quantity (1 part by weight) of (a) the anti- Again the physical properties ofthe'yulcafised'riibbers obtained were determined before ageing and (a) after ageing for 4 days at 70 C. in an oxygen bomb at a pressure of 300 lbs/sq. inch; (b) after ageing-for 8 -days-at oxidant mixture of Example 1, '(b') a similar antioxidant 5 in a P a w mixture withthe 2:4=dimethyl*6=octy1-phenol replaced by The following results were banned, as before: 2:6-dimethyl-4*octyl-phenol (obtained by the reaction 2:6-dimethyl-phenol with diisobutylene). 7

. Y en bomb test Again the physical properties of the vulcanised rubbers xyg obtained were determined before ageing and (a) after 10 TEWSIONISTRENGTH KGMS 3 CM ageing for 4 days at 70 C. in an oxygen bomb'at a .pres- (1) sure of 300 lbs/sq. inch; (b) after ageing 'for 8 days at 70 C. in a hot air oven. 7 Y After4 Peroeutage The following results Were obtained, those with the originaldays gfig first of the antioxidant mixtures agreeing Within the usual experimental variation with the results of Example 1: stock with antioxidant 85 Stock with antioxidant (b) 214 199 93 Oxygen bomb test n PERCENTAGE ELONGATION AT BREAK (i) TENSION STRENGTH (KGMs/SQ. CM.) 7 V a Stock with-antioxidant (11);. 745 680 91 After 4 Percentage Stock with antioxidant (b) 720 V 685 95 Original days of original 0 figure (iii) MODULUS AT 500% ELONGATION (KGMSJSQ. CM.)

Stockwithantioxidant (a) 233 204 87 v a. a, r l '0 w Stock with antioxidant (b) 230 189 82 V V v Ethel}; w n antioxidant (a) 71 '84 118 (ii) PERCENTAGE ELoNGATroN AT BREAK i 1 l l .1 m

Stock with antioxidant (a) 690 640 92 Stock with antioxidant (b) 685 625 91 Hot arr oven test (iii) MODULUS AT 500% ELCNGATICN (KGMSJSQ. CM.)

' (i) TENSION STRENGTH KGMs sq. CM.) Stock with antioxidant (a) 99 ;101 102 I Stock with antioxidant (b) 100 100 100 A Afters Percentage Original days 01' original figure Hot air oven test a 40 Stock with antioxidant-(a) 224 201 90 (i) TENSION STRENGTH KGMSJ5Q OM Stock with antioxidant (b) 225 193 86 After 8 eni (ii) PERCENTAGE ELoNGATroN AT BREAK Original days of original figure p .l s n Stock with antioxidant (a) 72 5 620 85 Stock with antioxidant (a) 233 183 78 Stock with antioxidant (b) 720 625 37 Stockwith antioxidant (b) .1 230 199 86 a. 7

(ii) PERCENTAGE ELONGATION AT BREAK (iii) MODULUS AT 500% ELONGATION (KGMSJSQ. OM.)

St kw'thantioxidant (a) 600 575 S3 with antioxidant 685 600 81 $335 at ifiii ii ifii E3231".- ii iii ii? (iii) MODULUS AT 500% EL'ONGATION (KGMs S CM.)

EXAMPLE 5 Stool: with antioxidant (a) 99 123 124 Stock with antioxidant (b) 100 123 123 This example illustrates the e'fiec'tivene'ss of the anti- PLE 4 oxidant mixtures even in the presence of a copper poison- EXAM 00 e'r, which greatly increased the tendency foioxidation 'of the rubber to occur.

The tests carried out in Example 1 were repeated us- In this example a rubber stock was made up as deing the same quantity (1 part by weight) of (a) the antiscribed in Example 1 except that 0.5 part by weight of oxidant mixture of Example 1, (b) a similar antioxidant copper oleate was included in the rubber stock as conmixture with the 2:4-dimethyl-6-octyl-phenol replaced by taminant. The stock was first made up without any antian octylated cresylic acid; in addition a mono-benzyl ether oxidant, and then with 1 part by weight of (a) the antiof a commercial mixture of dihydric phenols containing oxidant mixture of Example 1, (b) a similar antioxidant rather less catechol, but still a preponderance of catechol, mixture with the 2:4-dimethyl-6-octylphenol replaced by was used. 2:6-dimethyl-4-octyl-phenoi (obtained by the reaction The octylated cresylic acid was obtained by alkylating 2:6-dimethy1-phenol with diisobutylene).

a cresylic acid of boiling range 2i4224 C. (at atmos- Each of the stocks was vulcanised at 126 C. for 20 pheiic pressure) with diisobutylene at 130 C., using minutes, and the physical properties of the vulcanised rubsulphuric acid as catalyst. The octylated cresylic acid bers obtained were determined before ageing and after product had a boiling range of 130180 C. (at 9 mm. of ageing for 1 day at 70 C. in an oxygen bomb at a presmercury). 76 sure of 3001bs./sq. inch.

7 The following results were obtained:

Oxygen bomb test (i) TENSION STRENGTH (KGMSJSQ. CM.)

Percentage Original After 1 day of original figure Stock without antioxidant. 231 Sample me1ted Stock with antioxidant (a) 227 206 91 Stock with antioxidant (b) 224 113 50 (ii) PERCENTAGE ELONGATION AT BREAK Stock without antioxidant.-- Stock with antioxidant Stock with antioxidant (b) Sample mclted.- 635 (iii) MODULUS AT 500% ELONGATION (KGMSJSQ. CM.)

Stock without antioxidant 115 Sample meltecL- Stock with antioxidant (a) 112 108 96 Stock with antioxidant (11)...- 113 92 81 It will be seen that the antioxidant mixture containing 2:4-dimethy1-6-octyl-phenol was in this instance superior.

EXAMPLE 6 This example is similar to Example 5, illustrating the efiectiveness of the antioxidant mixtures in the presence of a copper poisoner. Herethetest of Example .5.was repeated, using 1 part by weight of (a) the antioxidant mixture of Example 1, (b) a similar antioxidant mixture with the 2:4-dimethy1- o-oct'yl-phenol replaced by the octylated cresylic acid indicated in Example 4, and using a mono-ether as indicated in Example 4.

The oxygen bomb test in this instance was continued for 2 days, and the following results were obtained:

Oxygen bomb test (iii) MZODULUS AT 500% ELONGATION (KGMSJSQ. CM.)

Stock with antioxidant (a) 98 Stock with antioxidant (b) 98 What is claimed is: 1. A sulfur-vulcanizable rubber containing an antioxidant comprising a mixture of (a) a 2:4:6-trialkylphenol selected from the group consisting of 2:4-dimethyl- 6-octyl-phenol, 2:6-dimethyl-4-octyl-phenol and 2:6-di- 8 butyl-4-methyl-phenol and (b) a mono-ether of a dihydric phenol wherein said ether is selected from the group consisting of benzyl and ethyl, and wherein the ratio of (a) to (b) is from 25:75 to :25 by weight.

2. A sulfur-vulcanizable rubber as defined in claim 1 wherein (b) is a mono-benzyl ether of a dihydric phenol. 3. A sulfur-vulcanizable rubber as defined in claim 1 wherein (b) is a mono-ethyl ether of a dihydric phenol.

4. A sulfur-vulcanizable rubber as defined in claim 1 wherein the ratio of (a) to (b) is 75:25 by weight.

5. A sulfur vulcanized rubber containing an anti-oxidant comprising a mixture of (a) a 2:4:6-trialkyl-phenol selected from the group consisting of 2:4-dimethyl-6-octylphenol, 2:6-dimethyL4-octyl-phenol and 2:6-dibuty-l-4- methyl-phenol and (b) a mono-ether of a dihydric phenol wherein said ether is selected from the group consisting of benzyl and ethyl, and wherein the ratio of (a) to (b) is from 25:75 to 75:25 by weight.

6. A sulfur vulcanized rubber as defined in claim 5 wherein (b) is a mono-benzyl ether of a dihydric phenol.

7. A sulfur vulcanized rubber as defined in claim 5 wherein (b) is a mono-ethyl ether of a dihydric phenol.

8. A sulfur vulcanized rubber as defined in claim 5 wherein the ratio of (a) to (b) is 75 :25 by weight.

9. A method of rendering a sulfur-vulcanizable rubber resistant to deterioration comprising incorporating in the rubber an antioxidant compn'sing a mixture of (a) a 2:4:6-trialkyl-phenol selected from the group consisting of 2z4-dirnethyl-6-octyl-phenol, 2:6-dimethyl-4-octyl phenol and 2:6-dibutyl-4-methyl-phenol and (b) a monoether of a dihydric phenol wherein said ether is selected from the group consisting of benzyl and ethyl, and wherein the ratio of (a) to (b) is from 25:75 to 75:25 by weight.

10. A method of rendering sulfur-vulcanized rubber resistant to deterioration comprising incorporating in the rubber an antioxidant comprising a mixture of (a) a 2:4:6-tria1kyl-phenol selected from the group consisting of 2 :4-dimethyl-6-octyl-phenol, 2: 6-dimethyl-4-octyl' phenol and 2:6-dibutyl-4-methyl-phenol and (b) a monoether of a dihydric phenol wherein said ether is selected from the group consisting of benzyl and ethyl, and wherein the ratio of (a) to (b) is from 25:75 to 75:25 by weight.

11. A method of rendering sulfur-vulcanized rubber resistant to deterioration comprising incorporating in the rubber stock prior to vulcanization and subsequently vulcanizing the'rubber an antioxidant comprising a mixture of (a) a 2:4:6-trialkyl-phenol selected from the group consisting of 2:4-dimethyl-6-octyl-phenol, 2:6-dimethyl- 4-octyl-phenol and 2:6-dibutyl-4-methyl-phenol and (b) a mono-ether of a dihydric phenol wherein said other is selected from the group consisting of benzyl and ethyl, and wherein the ratio of (a) to (b) is from 25 :75 to 75:25 by weight.

References Cited in the file of this patent UNITED STATES PATENTS Jones Oct. 18, 1938 

1. A SULFUR-VULCANIZABLE RUBBER CONTAINING AN ANTIOXIDANT COMPRISING A MIXTURE OF (A) A 2:4:6-TRIALKYLPHENOL SELECTED FROM THE GROUP CONSISTING OF 2:4-DIMETHYL6-OCTYL-PHENOL, 2:6-DIMETHYL-4-OCTYL-PHENOL AND 2:6-DIBUTYL-4-METHYL-PHENOL AND (B) A MONO-ETHER OF A DIHYDRIC PHENOL WHEREIN SAID ETHER IS SELECTED FROM THE GROUP CON SISTING OF BENZYL AND ETHYL, AND WHEREIN THE RATIO OF (A) TO (B) IS FROM 25:75 TO 75:25 BY WEIGHT. 